What is the Phenytoin (Epanutin) test?

The Phenytoin (Epanutin) blood test measures the concentration of phenytoin, an anti-epileptic medication used to prevent and control seizures. Monitoring blood levels is essential because phenytoin has a narrow therapeutic range.

Small changes in dose, absorption, or metabolism can lead to subtherapeutic levels or toxicity. Blood testing helps ensure the medication remains effective while minimising side effects.

This test is commonly used in people receiving long-term phenytoin therapy or when symptoms suggest under- or over-dosing.

Why do I need a test for Phenytoin (Epanutin)?

Phenytoin levels can vary significantly between individuals due to differences in metabolism, liver function, drug interactions, and adherence. Regular testing helps maintain seizure control while reducing the risk of adverse effects.

Testing is especially important when starting treatment, adjusting dosage, adding interacting medications, or if symptoms of toxicity or breakthrough seizures occur.

You may benefit from this test if you experience:

• Seizure recurrence or poor seizure control

• Dizziness or unsteadiness

• Slurred speech

• Confusion or drowsiness

• Nausea or vomiting

• Abnormal eye movements (nystagmus)

• Gum enlargement

• Suspected drug interactions

Benefits of the Phenytoin (Epanutin) test

1. Ensures effective seizure control
   Confirms drug levels are within the therapeutic range.

2. Reduces risk of toxicity
   Identifies dangerously high phenytoin concentrations.

3. Supports personalised dosing
   Helps tailor treatment to individual metabolism.

4. Monitors long-term therapy safety
   Useful for ongoing epilepsy management.

5. Detects medication interactions
   Identifies altered levels caused by other drugs.

6. Supports clinical decision-making
   Guides dose adjustments safely.

7. Useful in liver function changes
   Liver health affects phenytoin metabolism.

8. Improves treatment confidence
   Provides objective reassurance for patients and clinicians.

Step-by-step – how we offer Phenytoin (Epanutin) testing

Clinic-based testing (UK-wide)

• Choose your nearest UK clinic

• Attend your appointment

• A qualified clinician or phlebotomist collects your blood sample

• Sample is sent to the laboratory for analysis

At-home or hotel testing (+£60)

• A qualified phlebotomist visits your home or hotel

• Blood sample collected in a comfortable, familiar setting

• No clinic visit or travel required

Both services are available UK-wide.

Your Phenytoin (Epanutin) results explained

Low phenytoin levels

Low levels suggest insufficient medication in the bloodstream, increasing the risk of breakthrough seizures. Causes may include missed doses, poor absorption, or drug interactions.

Dose adjustment or further evaluation may be required.

UK reference range:

• Low: <10 mg/L

Normal phenytoin levels

Normal levels indicate that phenytoin concentration is within the therapeutic range, supporting effective seizure control with reduced risk of toxicity.

Clinical symptoms should still be considered alongside results.

UK reference range:

• Normal: 10–20 mg/L

High phenytoin levels

High levels indicate potential toxicity and increased risk of side effects. Symptoms may include confusion, poor coordination, nausea, or neurological changes.

Prompt clinical review is often required.

UK reference range:

• High: >20 mg/L